diff --git a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
index a314079379..52a7ab354f 100644
--- a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
+++ b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
@@ -358,7 +358,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 	}
 
 	@Override
-	public void setLocale(Locale locale) {
+	public void setLocale(@Nullable Locale locale) {
+		// Although the Javadoc for javax.servlet.ServletResponse.setLocale(Locale) does not
+		// state how a null value for the supplied Locale should be handled, both Tomcat and
+		// Jetty simply ignore a null value. So we do the same here.
 		if (locale == null) {
 			return;
 		}
diff --git a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
index fc294fe2b6..7418a28716 100644
--- a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
+++ b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
@@ -358,7 +358,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 	}
 
 	@Override
-	public void setLocale(Locale locale) {
+	public void setLocale(@Nullable Locale locale) {
+		// Although the Javadoc for javax.servlet.ServletResponse.setLocale(Locale) does not
+		// state how a null value for the supplied Locale should be handled, both Tomcat and
+		// Jetty simply ignore a null value. So we do the same here.
 		if (locale == null) {
 			return;
 		}